1. Field of the Invention
Embodiments of the invention generally relate to radio frequency (RF) signal amplification, and in particular, embodiments relate to the amplification of multi-carrier signals.
2. Description of the Related Art
Radio frequency power amplifiers are widely used to transmit signals in communications systems. Typically, a signal to be transmitted is concentrated around a particular carrier frequency occupying a defined channel. Information can be sent as a modulation of amplitude, phase, frequency, or combination of these that causes the information to be represented by energy spread over a band of frequencies around the carrier frequency. In many schemes, the carrier itself is not sent since the carrier is not essential to the communication of the information.
When a signal containing amplitude variations is amplified by a power amplifier, the amplified signal is distorted if the power amplifier does not exhibit a linear amplitude and phase transfer characteristic. When distortion is present, the output of the amplifier is not linearly proportional to the input. Distortion also occurs if (a) the phase shift introduced by the power amplifier is not linear over the range of frequencies present in the signal; or (b) the phase shift caused by the power amplifier varies with the amplitude of the input signal or vice versa (AM to PM or PM to AM, respectively).
The introduced distortion can include intermodulation of the input signal's components. The products of the intermodulation can appear within the bandwidth of the input signal causing undesirable interference. The intermodulation products can also extend outside the bandwidth originally occupied by the signal. Such out-of-band products can cause interference in adjacent channels and violate transmitter licensing and regulatory spectral emission requirements. Although filtering can be used to remove the unwanted out-of-band distortion, filtering is not always practical, especially if the amplifier is required to operate with multiple frequencies.
Distortion products that are at multiples of the carrier frequency (harmonic distortion) can also be produced in a non-linear amplifier. Harmonic distortion is relatively simple to remove by filtering.
Intermodulation can also be a problem when multiple signals are amplified in the same amplifier even if they individually do not have amplitude variations. A combination of multiple signals can produce amplitude variations as the various components beat with each other by adding and subtracting as their phase relationships change.
Power amplifiers can introduce some distortion even when well designed. Perfect linearity over a wide range of amplitude is impractical to realize in practice. In addition, as any power amplifier nears its maximum output power capacity, the output no longer increases as the input increases. At that point, the power amplifier is not regarded as linear. A typical power amplifier becomes significantly non-linear at a small fraction of its maximum output capacity. In order to maintain linearity, a power amplifier can be operated at an input and output amplitude that is low enough for the signals to be amplified in a part of the transfer characteristic which is substantially linear. However, with that type of operation, known as “backed off,” the power amplifier has a relatively low supplied power to transmitted power conversion efficiency. For example, a “Class A” amplifier may be linear enough to transmit a signal cleanly, but may be only 1% efficient. Low efficiency is wasteful of power and increases the size and cost of the power amplifier. Further, the power that is wasted is dissipated as heat, which has to be removed by cooling.
Communication schemes can include modulating constant-amplitude signals with frequency and phase modulation. These signals are relatively unaffected by distortion and can be amplified with highly non-linear amplifiers, which are smaller, cooler, more power efficient, and less expensive. Modulation of that type is used in, for example, conventional radio paging systems that use continuous phase frequency shift keying (CPFSK) modulation.
Bandwidth efficient modulation schemes typically use both amplitude and phase variation. In addition, users may transmit multiple signals on different channels, for example, different carrier frequencies, with a single power amplifier. That reduces the number of separate amplifiers used and avoids the need for large and costly high level output signal combining filters, which can have undesirable power losses.
Digital Predistortion
North American digital cellular telephony services employ linear modulation schemes to encode baseband information in both the amplitude and phase of an RF carrier for efficient bandwidth utilization. If significant intermodulation and distortion products are to be avoided, “class A” linear amplifiers can be employed. However, as described earlier, high-power linear amplifiers are generally inefficient and undesirable in systems in which cost and heat dissipation are prohibitive factors, for example, cellular telephone basestations, wireless access points, and the like.
To avoid the compromise of constraints between the regulatory spectral emission mask and amplifier efficiency, attempts have been made to harness the efficiency of nonlinear class AB power amplifiers by various linearization techniques. Analog feedback techniques have been reported, but these approaches can be limited to relatively narrow operating bandwidths, can be extremely sensitive to amplifier variations, and can be prone to instability. Consequently, these designs may not be appropriate for mass production. See, for example, NAGATA, Y., Linear Amplification Technique for Digital Mobile Communications, IEEE Vehicular Technology Conference (1989), pgs. 159-164; and CAVERS, J. K., Amplifier Linearization Using A Digital Predistorter With Fast Adaptation And Low Memory Requirements, IEEE Transactions on Vehicular Technology, Vol. 39, No. 4, pp. 374-383, November 1990.
Simulation work has postulated the advantage of adaptive digital feedback at baseband. Such simulation work indicates a relatively good reduction in out-of-band spectral emissions, typically in excess of 25 decibels (dB). These techniques are relatively insensitive to amplifier variations and provide an attractive design suitable for mass production. An adaptive complex gain predistorter achieves a reduction in out-of-band spectra in excess of 20 dB for a class AB amplifier operating close to saturation. See, for example, A. BATEMAN, D. M. HAINES, AND R. J. WILKINSON, Linear Transceiver Architectures, IEEE Proc. Veh. Technology Conf., Philadelphia, Pa. (1988), IEEE Catalog 2622-9/88/0000-0478, pp. 478-484; and R. D. Stewart and F. F. Tusubira, Feedforward Linearization of 950 MHz amplifiers, Inst. Elec. Eng. (IEE) Microwaves, Antennas and Propagation, Proceedings H, Vol. 135, No. 5, pp. 347-350, October 1988.
Complex Gain Predistortion
FIG. 1 illustrates a software/hardware configuration for an adaptive linearization circuit. In addition to the typical forward path components (digital-to-analog converter 102, quadrature upconverter 104, local oscillator 118) of a power amplifier 106, a feedback loop with an RF coupler 108, a quadrature downconverter 110, and an analog-to-digital converter 112 are present.
Signal designations refer to the complex baseband signals or the complex envelope of the bandpass signals. The illustrated notation is compatible with the original theoretical work of Cavers. A complex gain predistorter 114 generates a baseband complex modulation envelope Vd(t) that complements the nonlinearities introduced by the power amplifier 106. An adaptive estimator 116 compares a desired reference signal Vm(t) with an observed signal Vf(t) originating from the power amplifier 106, and estimates the complex gain predistortion coefficients. The measured complex modulation envelope Vf(t) is a scaled, rotated, and delayed version of the power amplifier output Va(t). The characteristics of the complex gain predistorter 114 are selected such that its non-linearity is complementary to that of the power amplifier 106. Further details of predistortion linearization based on FIG. 1 can be found in U.S. Pat. No. 6,356,146 to Wright, et al., the disclosure of which is hereby incorporated by reference in its entirety herein.
The adaptive algorithms employed in predistortion linearizers are typically intended to tune their parameters to minimize the total error (typically mean-squared error) between the desired reference signal Vm(t) and the observed signal Vf(t). However, the degree of linearization actually achieved can vary significantly according to both the instantaneous characteristics of the desired reference signal Vm(t) being predistorted and the amplifier's transfer characteristic. For example, those portions of the desired reference signal Vm(t) that exercise the saturated portion of the amplifier transfer characteristic (where its slope is approaching zero), cannot be effectively linearized due to the very large instantaneous corrective gain that would be applied by the predistorter 114. On the other hand, amplifier operation in the saturated region can be desirable for maximum efficiency with state-of-the-art amplifier designs. Assuming that the amplifier transfer characteristic is smooth and exhibits no other noninvertible regions prior to entering saturation, the overall linearity achievable with that amplifier is primarily defined by the rate and degree to which the desired reference signal Vm(t) exercises the saturation region of the amplifier's transfer characteristic. Recognition of the foregoing observation has led to two main approaches.
In one approach, the amplifier input signal is scaled downwards so that its peak power is statistically limited to some probability and level below saturation that is deemed acceptable, for example, from a spectral regrowth point of view, prior to predistortion. The output power backoff (OPBO) technique is effective in reducing the amount of nonlinearity, but typically incurs a relatively large loss of amplifier efficiency due to amplifier operation below saturation. Applying predistortion in that scenario can extend an amplifier's input range for a given level of acceptable distortion, but does not otherwise fundamentally improve upon the limitation posed by the maximum saturated output power of the amplifier.
In a second approach, the amplifier input signal is modified so that its crest factor, that is, its peak-to-average ratio (PAR) in amplitude or power, is decreased prior to predistortion. With crest factor modification, the relative frequency and amplitude of signal peaks are reduced so that the amplifier input signal can be scaled upwards to operate the amplifier closer to saturation for improved efficiency. Often, this crest factor reduction (CFR) is achieved at the expense of impairing in-band error vector magnitude (EVM), which can limit its application when EVM is subject to regulation. One disadvantage of conventional CFR is the additional power and complexity of the CFR hardware, along with the processing latency incurred (for time-critical systems).
U.S. Pat. No. 7,142,831 to Anvari is representative of a conventional approach in which the CFR circuit follows a predistorter. The two modules are controlled, operated, and have their respective parameters chosen separately rather than jointly. As such, overall system complexity and performance may often be suboptimal and can be improved.
U.S. Pat. No. 7,099,399 to McCallister describes a predistorter that provides a feedback signal that indicates the quality of overall system output so that a CFR circuit can modify its behavior to improve or meet its performance target (such as residual error). The CFR is a separate entity from the predistorter and their interaction is limited to a single feedback signal between the two entities, which requires that their respective adjustments be made sequentially rather than simultaneously, that is, jointly. Such sequential or “ping-pong” adjustments are generally suboptimal in final performance as well as convergence speed.
U.S. Patent Application Publication No. 2006/0229036 by Muller, et al., describes a predistorter responsive to, among other characteristics, a crest factor as an input. Muller's approach can be viewed as a feedforward counterpart to the feedback approach of U.S. Pat. No. 7,099,399 to McCallister and should be subject to the same limitations of complexity, final performance and convergence speed.
U.S. Patent Application Publication No. 2005/0157814 by Cova, et al. describes a crest factor of the predistorter input signal being used to select the signals to which the predistorter is adapted. By ensuring that such high crest factor portions of the predistorter input signal are adequately reflected in the choice of predistortion parameters, overall system performance can be improved relative to a predistorter that is adapted randomly over its input signal. Complexity, however, remains similar to that of a cascade of a CFR followed by a predistorter as two separate entities.